Focus distance help required

[OzEclipse]

Hi,

A friend of mine posted this question to another forum. Looking at his problem, the solution struck me pretty quickly.
I thought I'd share it here.

Joe
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Original Post
Focus distance help required
Hi everyone,
I need some help and hoping someone here will know the answer.

I have a Takahashi fsq-85 and I have replaced the focuser with an Esatto motor focuser which has only 30mm of travel. I am coupling it with a QHY 268c. I do have the Tak flattener 1.01x which I can use, but that was really only purchased because I formerly had a full frame camera. The fsq-85 is supposed to be good for APS-C sensors without the extra flattener.

The point of this thread though is that both with and without the flattener I can't bring the system to focus because there is insufficient travel. I do have 19mm of M54 screw thread extension tubes between camera and Esatto and the attached shows Acrux with the focuser fully extended.

Obviously I need extensions but we're not talking about a little off here, as you can see Acrux is massively way off focusing. I can have an extension piece made, but I do need to know how long it should be.

Is anyone using this combo and can therefore let me know how you reach focus?


My response
Hi Chris,

If the image you posted is an uncropped APS C, then the defocussed star diameter is about 15.7mm depending upon the specifics of your APS C sensor and blurring at the edges.

If the FSQ 85 with or without the flattener is the model with the 450mm focal length, then without any reducer this is a simple geometric similar triangles calculation:-

Extension required is approximately
15.5mm x 450mm÷85mm = 82mm

Even using the 1.01 field flattener, the ray trace convergence will barely be diverted, maybe by about 4.5 mm

I would retake the image with the Estatto at mid-travel rather than at one end measure the out of focus image and repeat the calculation.

Cheers
Joe